Glyphosate (International Standards Organization common name for the anionic form of N-phosphonomethylglycine) is widely used as a postemergence foliarly applied herbicide.
Until recently glyphosate could only be used to control weeds in a non-selective manner, i.e. if applied to foliage of a standing crop it would kill the crop as well as the weeds. Very low rates of postemergence foliar application of glyphosate have been shown to allow the control of parasitic weeds on standing crops (Foy et al., 1989), but the rate dependence was so crucial and hard to adjust that this use has not been widespread. This use though was predicated on the well known fact that glyphosate is systemic once applied to plant foliage. The ability of glyphosate to act solely through foliage is well accepted in agriculture, as it is well known that it is rapidly degraded in all natural soils, i.e. soils that have not been sterilized (Rueppel et al., 1977). It thus cannot be used as a soil-applied "preemergence" herbicide to control weeds underground, unlike so many commonly used herbicides in agriculture.
As noted above, glyphosate has been used as a non-selective herbicide until recently. That has been changed with the advent of glyphosate-resistant crops. This change has been achieved by engineering a gene coding for enolphosphate shikimate phosphate synthase also known as 5-enolpyruvyl-3-phosphoshikimate (EPSP) synthase, an enzyme that is found in, and is vital to, all plants, and is inhibited by glyphosate. The transgenic plants either contain multiple copies of the natural gene (Rogers et al., 1983) or a modified gene coding for an enzyme that still has a modicum of natural enzyme capacity but is severely reduced in its capacity to bind glyphosate and thus be inhibited (Comai et al., 1985). The gene must code for a modified form of the EPSP synthase, whereby this enzyme is less inhibited by glyphosate than the naturally-occurring enzyme, or for elevated expression of the enzyme, or both.
Such engineered glyphosate-resistant plants often contain subsidiary genetic elements that target the modified gene product to the chloroplasts, and control the level and tissue-localization of the gene (Shah et al., 1986; Kishore, et al., 1988). Examples of such transformed plants are described in U.S. Pat. No. 5,145,783, EP 550633, U.S. Pat. No. 5,310,667 and U.S. Pat. No. 4,971,908, all of them incorporated herein in their entirety by reference. In addition, glyphosate resistance can also be achieved when a plant is able to amplify the gene (Suh et al., 1993) or to enhance the expression of the gene (Hollander-Czytko et al., 1992) coding for EPSP synthase.
Parasitic weeds can either be holoparasites, i.e. plants totally lacking the capacity to produce nutrients for themselves, e.g. Orobanche spp. (common name: broomrapes), or hemiparasites, i.e. they can perform photosynthesis for parts of their life cycles (e.g. Cuscuta spp. (dodders), Striga spp. (witchweeds) and Alectra spp.), but derive much of their organic nutrition, water and minerals from the host plants. The Cuscuta spp. attach to stems and grow above ground, the others attach to roots and spend much of their life cycle below ground until a flower stalk emerges from the soil. One of the major modes of dissemination of parasitic weeds is by contamination of crop seed. Half of the seedlots sampled in local African markets by Bemer et al., 1994 were contaminated with Striga seeds. Orobanche seeds stick to crop seeds and arduous procedures are required to remove them so as not to infest uninfested fields. Thus, a good general topical disinfectant is needed for inactivating parasitic weed seeds in contaminated seedlots prior to sowing. Additionally, there is also a general need for ridding crop seed of other contaminating non-parasitic weed seeds.
Parasitic weeds are a scourge threatening 4% of cropland worldwide, infecting all grains cultivated south of the Sahara (witchweeds=Striga spp) and vegetables, legumes and sunflowers (broomrape=Orobanche spp.) in the Mediterranean, including Israel. The yield loss (on the average) is more than 50% in the infested fields. Till recently there were few selective herbicides capable of controlling the root parasitic weeds while they are still underground, perpetrating their damage.
It has been shown that a foliar application of glyphosate to transgenic plants of the type discussed above allows the systemic inactivation of parasitic weeds (Joel et al., 1995), as had been predicted earlier (Gressel, 1992). It has also been shown that soil-active herbicides at very low rates can be applied to seeds of cow peas, known to be capable of degrading particular soil-active herbicides, in order to control parasitic Striga. Striga has also been controlled at much higher rates in maize with biotechnologically-derived resistance to the same groups of soil-active herbicides (Ransom et al., 1995). Glyphosate is far superior to these other herbicides insofar as weeds are far more prone to evolving resistance to the other herbicides previously used.
None of the prior art publications disclose or suggest the use of the foliar herbicide glyphosate in the soil or for seed dressing.